Process for stabilizing dithioglycerols



FLOW TIME (SECONDS) July 1, 1947.

G. w. RIGBY 2,423,344

PROCESS FOR STABILIZING DITHIOGLYCEROLS Filed Aug. 16, 1945 200 TIME (MINUTES) 6601:?6 INVENTOR.

WA TTOR NE Y Patented July 1, 1947 PROCESS FOR STABILIZING DITHIOGLYCEROLS George Wayne Rigby, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application August 16, 1943, Serial No. 498,846

This invention relates to thioglycerols, in particular to 1,2-dithioglycerol. More specifically, it relates to a process for improving the stability of thioglycerols against thermal decomposition.

2,3-dimercaptopropanol-1, hereinafter called 1,2-dithioglycerol, is a recently synthesized compound of considerable interest and usefulness, either per se, e. at, as softening agent, plasticizer, etc., or as an intermediate in chemical syntheses because of the presence, in the same molecule, of an hydroxyl group and two vicinal mercapto groups. There, is, however, one property of the thioglycerols which has militated aaginst their extensive use. This is their heat instability. This thermal instability, which Was observed by the first investigator in the field iCarious, Ann. 124, 257 (1861)] is particularly noticeable with 1,2-dithioglycerol. The problem it presents is two-fold. First, distillation oi crude 1,2-clithioglycerol on anything approaching a commercial scale presents extreme dimculties. Even using very small batches, very low pressure, and inert atmospheres, distillation of 1,2-dithioglycerol entails considerable losses in the form of undistillable'residue. Large quantities cannot be distilled in a single operation, since, after a small amount of distillate has been collected, the residue resiniiles and the bulk f the material is lost. Second, the distilled 1,2-dithiglycerol is itself also heat-unstable and, in reactions or uses involving a high temperature, de-

composes to a large extent with formation of undesirable by-products and the attendant disadvantages of low yields, difficult reaction control and wasted material.

The heat decomposition of the thioglycerols is particularly severe when they are prepared by reacting the corresponding glycerol halohydrins with alkali hydrosulfldes. The usefulness of this preparative method, which is intrinsically the most convenient and preferred one, is thus largely negated by the fact that the reaction product can be isolated only with great difficulty and, when isolated, is still thermally unstable.

This invention has an object the provision of a process for stabilizing thioglycerols, in particular 1,2-dithioglycerol, against thermal decomposition. Another object is to provide a process whereby 1,2-dithioglycerol can be distilled in high yields and with a minimum Of side reactions from crude reaction mixtures containing it. A further object is to stabilize distilled 1,2- dithioglycerol against further heat decomposition. Other objects will appear hereinafter.

These objects are accomplished by the follow 1'7 Claims. (CL 260-609) with the thioglycerol, and particularly 1,2-dithioglycerol, as a stabilizer against thermal decomposition, a member of the class consisting of ammonia, ammonium salts and amides. The invention also comprises thioglycerol compositions and particularly 1,2-dithioglycerol compositions,

having a thermal decomposition factor, K (as defined hereinbelow), of not more than 5 10-:.

According to the preferred mode of practicing the invention, there is added to the 1,2-dithioglycerol, prior to the heat treatment (e. g., distillation, high temperature reaction, etc.) about 1% by weight of aqueous ammonia, ammonium ace tate or ammonium phosphate. This simple treatment is suillcient to decrease considerably the tendency of 1,2-dithioglycero1 to decompose, resinify or dehydrate when heated. When the treatment is applied to the crude product obtained by reacting glycerol-1,2-dibromohydrin with sodium hydrosulfide, it greatly facilitates the isolation of the pure product since the crude 1,2.-=

dithioglycerol so obtained cannot be distilled (except on an impractically small scale) without the use. of one of the stabilizers of this invention. The mode of operation of the stabilizers proposed herein has not been-satisfactorily explained. Their beneficial action is apparently not a result of the neutralization of the acidic components which may be present in crude 1,2- dithioglycerol, since bases such as piperidine or sodium bicarbonate are not only ineffective, but definitely harmful in that they actually increase the rate of thermal decomposition. Conversely, amides as a class, although not particularly basic, are excellent stabilizers.

The thioglycerols may be made by methods published in the literature, preparation of 1,2-dithioglycerol is described in Ber. 75, 13 (1942). As mentioned before, one of the most economical processes, which is also the process in which the heat decomposition is the most serious problem, is thatin which a glycerol dihalohydrin is reacted with sodium hydrosulfide.

The 1,2-dithioglycerol used in the following exampl and tests was prepared as follows: Five hundred and sixty parts of a 15.6% methanol solution of sodium hydroxide are saturated with hydrogen sulfide at 0 C. and charged into a cooled steel autoclave together with 145 parts of glycerol- 1,2-dibromohydrin. The reaction mixture is then saturated promptly with hydrogen sulfide at 100 lbs/sq. in. pressure and the mixture is heated at C. for 5 hours. The contents of the autoclave,

For example, the

assess:

are cooled and acidified with 8% hydrochloric acid and the methanol is then removed by distillation at reduced pressure. The residue of oil and salt is taken up in water and chloroform,

the layers are separated and the aqueous solu-- many forms of the invention other than these specific embodiments.

ExAMPLr:

To 600 parts of topped dithioglyceroi. i. e., the crude reaction product left after removal of the inorganic salts and the volatile constituents as described above, are added 6 parts of ammonium acetate. The mixture is then charged into a still pot which is heated in an oil bath at a temperature of 150--l70 C. and the 1,2-dithioglycerol is distilled at a pressure of 15 mm. After a small foreshot amounting to about three parts the major part of the product distills at a temperature of .110-120 C. The distilled dithioglycerol is of high purity as shown by its physical constants:

Viscosity 13.5 centipoises at 25 0. H2O solubility 7.8% at 25 C.

Surface tension 46.9 dynes/cm. at 25 C.

U. V. transmission-.. 86% through a 1 cm. layer at wave-length 3341, with the instrument set so that 1 cm. water transmits 100%.

physical constants:

H2O solubility--- 6.00% at 25 C.

Surface tension 40.5 dynes/cm. at 25 C.' U. V. transmission.. through a 1 cm. layer at wave-length 3341, with the instrument set so that 1 cm. of water transmits 100%.

and 202 parts of very viscous, non-voltatile residue. The distilled dithiogiycerol amounts to 53% of the material charged and the residue comprises 34% of the material charged. This shows that, in the absence of the added ammonium acetate stabilizer, the distilled product is of poor. quality 4 s If a'sample of 1,2-dithioglycerol is heated under an inert (e. g., nitrogen) atmosphere in a glass tube at constant temperature, th relatively viscosity of the material can be determined at intervals by means of a simple viscgmeter, for example, a glass pipette. This viscometer is kept immersed in the liquid until a test is made. To make a measurement, the viscometer is filled with the liquid. and raised above the surface. The time required for the liquid in the viscometer to flow, under the influence of gravity, between two graduations, on the viscometer. is noted. .11 constant temperature for the heating and the vis-.

cosity measurement is conveniently obtained by placing the tube containing the 1,2-dithioglyceroi and the viscometer in a glass jacket where it is surrounded by the vapors of a constant boiling, stable liquid maintained at its boiling point. Such a liquid may be, for example, bromobenzene. which boils at 156 C. The relative flow times, i. e., the'ratios between the flow times at various time intervals and the initial flow time (all being determined at the same temperature) are independent of the viscometer used and give an accurate picture of the increase in viscosity, which in turn is an index of the progressive decomposition of the sample. For example, a sample of distilled 1,2- dithioglycerol had an initial flow time, in the viscometer employed, of 109 seconds at 156 C. After two hours at that temperature, the flow time was 157 seconds; after four hours, it was 301 seconds and, after five hours, 404 seconds. In contrast to this, distilled 1,2-dithioglycerol containing 1.25%

. byvolume of commercial ammonium hydroxide (ammonia) and which showed an initial flow time 7 at 156? C. of 105 seconds, had after seven hours and is obtained in lower yield than in the presence oi the stabilizer.

The remarkable increase in the heat stability of 1,2-dithioglycerol treated according to the method of this invention can readily be shown by a simple viscosity test as follows: On heating, 1,2- dithioglycerol undergoes decomposition and progressive polymerization which manifest themselves by an increase in the viscosity of the liquid.

heating at that temperature a flow time of only 108 seconds. This shows the remarkable increase in stability resulting from the addition of a small amount of aqueous ammonia.

The influence on the heat stability of 1,2-dithioglycerol of small amounts of the stabilizers proposed herein can be shown graphically by plotting on semi logarithmic paper the heatin times in minutes as abscissae and the flow times in seconds as ordinates. It is found that, after an induction period, the curve correlating heating times and flow times for, eachsample is essentially a straightline.

The drawing shows a set of five such curves. The middle one is that of distilled 1,2-dithioglycerol alone, i. e., the control. 'The considerable heat stabilizing effect of urea and ammonium chloride (1% by weight of the dithioglycerol) is shown by the two lower curves. For comparison, the two upper curves show the harmful effect caused by the use of 1% by weight of two organic bases, n-octylamine and piperidine. All measurements were made at 156 G.

Since the viscosity curve for any given sample of 1,2-dithioglycerol is a straight line (neglecting the induction period), its slope is a constant which expresses the heat stability of the sample. This constant, K, which may be called the thermal decomposition factor, is obtained by dividing the difference between the logarithms of the flow times (in seconds) at two different times T: and T1, by the difference (in minutes) between the same two times:

T -log flow time at time T,

log flow time at time distilled 1,2-dithioglycerol is 22.8X10 Samples of greater stability have a smaller factor,

' while less stable samples have a higher factor. I The following tables showthe increase in heat stability resulting from the addition of 1% by weight of ammonia, various ammonium salts and various amides to distilled 1,2-dithioglycerol:

For comparison, it is interesting to note that a number of acid-accepting agents not belonging to the class of ammonia, ammonium salts and amides haves definitely harmful efl'ect on the heat stability of 1,2dithioglycerol, as shown by the following table. In all cases there was added 1% by weight of acid-accepting agent.

Table II Acid-Accepting Agent (1% by weight) K at 166 C.Xl-

None 22. 8 Piperidine 62. 1 Sodium bicarbonate.-. 47. 4 Triethanolamine 44. 4 Sodium methylate. 43. 0 Ethanolamine 29. 0 n-Octylamine 25. 6

The above tables refer to 1,2-dithioglycerol which has been previously distilled. The following table shows the data from viscosity measurements on a sample of stripped crude 1,2-dithioglycerol, i. e., the crude reaction product from which the inorganic salts and volatile material have been removed.

Table III Stabilizer (1% by weight) K at 156 (LX10 None 160. 00 Ammonia 4. l9 Glycine plus ammonia 3. l4 Urea Ammonium acetate Hydrogen diammoninm phosphate 'Inammonium phosphate Disodium ammonium phosphate-.-

As indicated before, the unstabilized crude re action product cannot be distilled except in very small batches and under very low pressure. Ta-

ble III shows the remarkable improvement in the heat stability of crude 1,2-dithioglycerol, as expressed by the values of K, brought about by various stabilizers.

Any organic or inorganic ammonium salt, or any carbonamide, is operable in the process of this invention. In addition to the stabilizers already disclosed, there may be mentioned ammonium fluoride, ammonium bromide, ammonium iodide, ammonium carbonate, ammonium sulfamate; the ammonium alts and amides of the aliphatic acids such as propionic, lactic, butyric, valeric, lauric, palmitic, stearic, undecylenic, oleic, and sorbic acids; -of the mixed fatty acids occurring naturally in oils such as fish oil or linseed oil; of cycloaliphatic acids such as cyclohexanemonocarboxylic and cyclohexanedicarboxylic acids: of aromatic acids-such as benzoic,

toluic, and naphthoic acids; of araliphatic acids such as phenylacetic, phenylpropionic and cinnamic acids; of polybasic acids such as oxalic, adipic, sebacic, and tartaric acids; of heterocyclic acids such as furoic and nicotinic acids;

the N-monoand disubstituted amides such as N-methylacetamide, N-ethylpalmitamide, N,N- dimethyiforma'mide, N,N-diethylacetamide, ethyleneurea, N,N'-dimethylurea; cyclic amides such as epsilon-caprolactam; inner ammonium salts such as glycine; semi-carbazide, either as such or as the hydrochloride; biuret, etc. Mixtures of two or more stabilizers, such as ammonium hydroxide and ammonium phosphate, can be used, if desired. It is unnecessary that the stabilizer be soluble in the 1,2-dithioglycerol to any substantial extent.

0f the broad class of stabilizer disclosed, the preferred ones are those which bring the thermal decomposition factor of 1,2-dithioglycerol down to a value of 5 10- or less. Dithioglycerol having a thermal decomposition factor of less than 5x 10- is particularly useful since it is quite satisfactory for use in reactions involving high temperatures. Because of their cheapness and effectiveness, ammonia and ammonium phosphate are two of the preferred stabilizers.

The stabilizers can be used in any desired amount, but it is obviously uneconomical to use more than is required for adequate stabilization. In general, it is quite unnecessary to use more than 5% by weight, and 1% has been found sufllcient in the large majority of cases. Prefer ably, there should be used at least 0.1% of stabilizer by 'weight of 1,2-dithioglycerol, though quantities as small as 0.01% have a beneficial eflect.

In addition to ammonia, ammonium salts and amides, a few other compounds have been found to influence favorably the heat stability of 1,2 dithioglycerol. Some of these compounds are methylamine, hexamethylenetetramine, by droxylamine, tributylamine and a mixture of disodium hydrogen phosphate and potassium dihydrogen phosphate. However, the eflect of these materials is usually not as marked as that oi the ammonium salts and amides, and further more, such effect as they show is not general for the chemical classes to which these agents belong.

This invention has been illustrated with particular reference to 1,2-dithioglycerol. However, it is of value also with the other thioglycerols, i. e., monothioglycerol, 1,3-dithioglycerol and tri thioglycerol.

The above description and examples are in tended to be illustrative only. Any modification. of or variation therefrom which conforms to the spirit of the invention is intended to be included within the scope of the claims.

What is claimed is:

1. A dithioglycerol composition containing as a thermal stabilizer therefor a member of the class consisting of ammonia, ammonium salts, and carbonamides, said composition having a thermal decomposition factor of not more than 5 1O- said thermal decomposition factor being equal to log flow time at time T' -log flow time at time T; 1

2. A 1,2-dithioglycero1 composition containing as a thermal stabilizer therefor a member of the class consisting of ammonia, ammonium salts,

and carbonamides, said composition. having a thermal decomposition factor of not more than X10 said thermal decomposition factor bein equal to log flow time at time T,1og flow time at time T 3. A 1,2-dithioglycerol composition containing ammonia, said composition having a thermal decomposition factor of not more than 5X said thermal decomposition factor being equal to T,-log flow time at time T Ta- 73 4. Process of stabilizing a dithioglycerol which comprises incorporating therewith a member of the class consisting of ammonia, ammonium salts and carbonamides.

log flow time at time 5. Process of stabilizing 1,2-dithioglycerol' which comprises incorporating therewith a member of the class consisting of ammonia, ammonium salts and carbonamides.

6. Process of stabilizing 1,2 dithioglycerol which comprises incorporating ammonia therewith.

7. A dithioglycerol composition containing as a thermal stabilizer therefor a member of the class consisting of ammonia, ammonium salts, and carbonamides.

8. A 1,2-dithioglycero1 composition containing as a thermal stabilizer therefor a member of the class consisting of ammonia, ammonium salts, and carbonamides.

9. A 1,2-dithioglycerol composition containing ammonia.

. as a thermal stabilizer therefor, ammonium acetate.

16. A 1,2-dithioglycerol composition containing, as a thermal stabilizer therefor, a carbonamide.

17. A 1,2-dithioglycerol composition containing, as a thermal stabilizer therefor, urea.

GEORGE WAYNE RIGBY.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,926,797 Sutton Sept. 12, 1933 2,130,321

Kharasch Sept. 13, 1938 

